IONOSPHERE RESEARCH AND PROPAGATION The Bureau conducts and coordinates basic research on the propagation of radio waves as affected by the ionosphere and on the special factors which can give rise to large departures from the normal behavior; conducts basic research on the nature of the media through which these radio waves are transmitted and the interaction of radio waves with the media; prepares predictions of radio wave propagation and warnings of disturbances; acts as a central repository for data, reports, and information in the field of ionospheric radio wave propagation; and provides consultation services on the characteristics of the ionosphere and on radio wave propagation to other government agencies and industry. Javelin rocket which carried an ionosonde to an altitude of 1,000 kilometers. For thirteen minutes, the ionosonde made topside soundings of the ionosphere. The experiment was made to test the sounding system for the fixed-frequency satellite topside sounder to be orbited in 1962 (page 127). VLF Phase Stability Studies. Propagation characteristics of phase stabilized transmission in the very low frequency (VLF) band of the radio spectrum are studied to provide data on the state of the lower ionosphere. A record of the phase and amplitude of a VLF transmission discloses the normal variation in the phase resulting from changes in the effective height of reflection of the ionosphere from day to night. Abnormal variations in the phase of a VLF signal are observed to accompany both magnetic storms and solar flares (Sudden Phase Anomalies). An unusually large Sudden Phase Anomaly (SPA), produced by a solar cosmic ray flare, was observed at 1030 UT, 4 May 1960 on the record of the 16 kc/s GBR transmission from Rugby, England, to Boulder, Colo. The high energy cosmic ray particles produced by this solar flare ionized a region well below the normal D-region of the ionosphere, thus producing the large SPA. (Only 10 such solar flare cosmic ray events have been observed in the period from February 1942 to December 1960.) In addition, analysis of the 16 kc/s data has revealed variations in phase coincident with meteor shower activity. The use of the phase-coherent detection technique at VLF is apparently a sensitive indicator of the worldwide ionizing effects of meteors which are not as easily seen in smaller volume samples, such as those obtained by VHF forward scatter, radar, and optical observations. Magnetic Field Micropulsations and Electron Bremsstrahlung. Enhanced magnetic micropulsation activity in the auroral zone near College, Alaska, has been observed simultaneously with increases in electron bremsstrahlung intensity. Measurements of the magnetic field fluctuations were made with a 2-m-diam loop antenna of 21,586 turns with its axis in the magnetic North-South direction. The system had a flat response to magnetic flux density in the frequency range of 0.4 to 0.04 c/s. Bremsstrahlung from bombarding electrons having energies greater than 50 kev were observed with balloon-borne Geiger counters. X-ray bursts, representing high energy electron influx, were found to be coincident with the magnetic field micropulsation amplitudes. Balloon measurements of electron bremsstrahlung using rapid time response scintillation counters give some indication of the exist ence of bunching in the incoming electron density. Further observations should show whether such fluctuations are also simultaneous with the magnetic field pulsations. Ray Tracing Through The Real Ionosphere. Ionospheric data collected during the IGY show that large regions of high electron density exist in the vicinity of the magnetic dip equator. Under such conditions the assumption of a spherically stratified ionosphere is invalid and it becomes. necessary to plot the radio wave ray paths in detail so as to ascertain the modes of propagation. Cross sections, along the 75° west meridian, have been made for noon and evening conditions in March 1958. Neglecting the earth's magnetic field, rays have been constructed from a point located at 20° North latitude for several frequencies and angles of elevation. Mountain-top transmitting location for studies of fading characteristics in wide-band transmission systems. Receivers are located at a distance of 70 miles on the plains within the line-of-sight. Problems of space telecommunications have led to a need for more data on long line-of-sight transmission paths (page 128). Of particular interest is the presence of supermodes. These involve ray paths that undergo two successive reflections from regions of high electron concentration without the usual ground reflection in between. In this way signals can propagate at frequencies considerably higher than the classical maximum usable frequency. Doppler Fading Studies. Over the past year a sensitive technique has been developed for the recording of the frequency variation of ionospherically propagated radio waves. These frequency variations are of the order of a few cycles per second and for purposes of analysis they are recorded on magnetic tape traveling at a speed of 150 ips. By playing the tape back at a speed of 30 ips the frequency variations appear as an audio tone which can be analyzed by conventional techniques. The 20 Mc/s transmission of WWV has been recorded at Boulder for a number of months in the winters of 1959-1960 and 1960-1961. Within a minute of the optical onset of a large solar flare of November 12, 1960, the frequency started to increase. This increase can be interpreted as a downward motion of the reflecting level. Later the ionosphere stopped moving and the frequency returned to normal. This technique can also be used to identify magnetic storm effects and traveling disturbances in the F region. During a severe magnetic storm it has been observed that the F layer loses its specular reflection character and degenerates into a turbulent medium. First Rocket-Borne Soundings of the Topside of the lonosphere. Shortly after 1800 EST on June 24, 1961, a two-frequency ionosonde was carried to an altitude of over 1000 km off Wallops Island, Va., by means of a four-stage rocket (Javelin). Successful radio pulse reflections from the topside of the ionosphere were obtained for about 13 of the 15 minutes that the payload was above the height of the F region maximum electron density. The experimental technique involved is essentially the same as that used by the network of bottomside sounders except that the rocket-borne sounder was completely transistorized and operated at a lower power. Unexpected effects were observed as the sounder passed through levels in the ionosphere where the plasma frequency was equal to the sounding frequency. Also, some evidence for the presence of ionization irregularities at altitudes of 700-900 km was obtained. A preliminary analysis of the rocket results suggests a neutral atmosphere scale-height of about 70 km between the altitudes of 400 and 600 km, implying a temperature of about 1,200 °K (assuming an oxygen atmosphere). The purpose of the rocket experiment was to test the sounding system that is to be used in a fixed-frequency satellite topside sounder scheduled to be placed in orbit in 1962. NBS responsibilities in this program include overall planning, design and performance of the experiment, and analysis of the resulting data. Airborne Instruments Laboratory, a division of CutlerHammer Company, is designing and building the rocket and satellite payloads and the ground data handling equipment. Technical management and sponsorship is provided by the National Aeronautics and Space Administration (Goddard Space Flight Center). Studies of the Interplanetary Medium. A study of the relation of solar emission of medium-energy particles to other types of solar activity has revealed new facts about the interplanetary medium. These particles, first suspected in the large solar event of February 1956, have been detected in the earth's atmosphere and their effects studied by means of VHF forwardscatter signals. These data, and others measuring the ionospheric effects of the solar particles, show that around the time of maximum solar activity the solar cosmic ray particles take much longer to reach the earth from the sun than they do near minimum activity. Comparison with characteristics of solar particles of higher and lower energies show that these mediumenergy particles must move in the interplanetary magnetic field not as single particles, but as a group. Consideration of directly-observed energy spectra of the various solar particles show that this group behavior is to be expected if the interplanetary field is regular but weak near solar minimum activity, and contains regions where the magnetic field intensity is 10 or 10-5 gauss near the maximum of the solar activity cycle. Linear dimensions, field strength, and frequency of occurrence of these regions of enhanced magnetic field, estimated from the behavior of the solar particles, are found to be consistent with the hypothesis that the clouds are formed through the action of low-energy solar particles. The effect of these outward-moving magnetic clouds on the velocity distribution of cosmic rays accounts for the main features of solar modulation of cosmic rays. Radio Reflections from Artificial Electron Clouds. In a cooperative program with the Air Force Cambridge Research Laboratory, the Bu reau has been involved in the operation of strategically located ionosondes during rocket experiments aimed at the creation of electron clouds. In these experiments relatively small amounts of various substances (such as sodium) are injected into the atmosphere at ionospheric heights. In one class of experiments, clouds of free electrons result through the ionization of the ejected material by sunlight. By permitting a measurement of the radio distance to ionized clouds over a wide range of radio frequencies, the ionosonde is a valuable tool for the study of cloud position, drift, and growth. Studies of the drifts of these clouds allow determination of wind velocities and the height gradients. For example, a value of about 7m/s/km was deduced for the East-West height gradient of the drift speed at 100 to 120 km during July-August 1960 over northern Florida. RADIO PROPAGATION ENGINEERING More efficient use of the radiofrequency spectrum is the aim of the Bureau's program in radio propagation engineering. This objective requires a basic understanding of radio wave propagation, noise, and interference. To this end theoretical and semi-empirical prediction methods are developed and compared with statistical samples of data on radio wave propagation and radio noise. During the past few years it has been found desirable to increase emphasis on the direct measurement of the characteristics of the atmosphere which affect the propagation. Further emphasis is also being given to studies of propagation and noise at the two extremes of the useful spectrum: above 5,000 Mc/s and below 30 kc/s. In this way it is hoped that a better understanding of these portions of the spectrum will lead to their more extensive and efficient use. New methods were Tropospheric Transmission Loss Predictions. published in the past year for predicting the transmission loss expected on a point-to-point tropospheric radio circuit. These thus provide an accurate basis for designing such radio systems. The accuracy of these prediction methods is such that costly path loss measurements prior to the installation of such a communications system are no longer necessary: These measurements can often be misleading unless they are made over a sufficiently long period of time, which in some cases may be several years. Wideband Data Transmission. Current use of radiofrequencies, with the transmission of large amounts of information, often involves very wideband systems. The advent of space telecommunications and highaltitude space vehicles has made it necessary to consider very long line-ofsight paths involving transmission through the troposphere. A measurement program has been initiated to investigate the maximum effective bandwidth that the troposphere can support without serious distortion due to multipath effects both within and beyond the line-of-sight. The within-lineof-sight observations have been made from a mountain site in Colorado |